Abstract

This presentation will review the late 20th century climate change over the
Tibetan Plateau. Studies, including this one, suggest an elevation
dependent warming on the plateau, i.e. higher warming at higher elevation.
I will present analyses of observed climate variables (1961-2000) and a GCM
output to discuss some of the mechanisms responsible for this phenomenon.
In particular, I will focus on the localized greenhouse effect of water
vapor increases in the boundary layer, which will be referred as surface
water vapor feedback (SWVP). SWVP appears to be the major cause of surface
warming during winter, particularly at higher elevations, in the model.
This is supported by greater increases in downward longwave radiation
relative to outgoing longwave radiation at surface during winter. This
difference is greater at higher elevations. Seasonally, observations show
largest warming trend in winter when there are much greater increases in
the minimum temperatures relative to the maximum temperatures. These
differences are also greater at higher elevations. Cloud cover, which can
significantly influence the downward longwave flux, show decreasing trends
in both observations and the modeled results. Between 1950 and 2100, the
model shows an elevation dependent warming trend during winter and spring.
We find that (1) increases in downward longwave radiation influenced by
increases in surface specific humidity during winter, and (2) increases in
absorbed solar radiation influenced by decreases in snow cover extent
during spring are, in part, the reasons for a large warming trend over the
plateau in the model.